1. Technical Field
The present invention relates to a projection optical system, an exposure apparatus, and a device manufacturing method, and relates, for example, to a reflecting type projection optical system for x-ray projection exposure apparatuses that use x-rays to transfer a circuit pattern on a mask onto a photosensitive substrate by means of a mirror projection system.
2. Background Art
Conventionally, an exposure apparatus that uses x-rays has gained attention as an exposure apparatus used in the manufacture of semiconductor devices, etc. If x-rays are used as the exposure light, there are only limited choices of transmitting optical materials and refracting optical materials, so in addition to using a reflecting type mask, a reflecting type projection optical system is used. Conventionally, a 4-mirror reflecting type optical system or a 6-mirror reflecting type optical system comprising four or six reflecting mirrors and having an image side numerical aperture of approximately 0.1 to 0.2 have been proposed as a projection optical system that can be adapted to exposure apparatus that uses x-rays as the exposure light.
In addition, recently, an 8-mirror reflecting type optical system that comprises eight reflecting mirrors and has an image side numerical aperture of 0.3 or more has been proposed as a projection optical system that can be adapted to an exposure apparatus that uses x-rays as the exposure light, for example, in the specification of U.S. Pat. No. 5,686,728 and in the specification of U.S. Pat. No. 6,710,917.
In a conventional 8-mirror reflecting type optical system described in the above patents, the width dimension (dimension along the radial direction of a circle centering on the optical axis) of an arcuate effective image forming region (a region in which, at the image surface, aberration has been corrected to the desired status) on the image surface is a maximum of approximately 2 mm. When the width dimension of the arcuate effective image forming region is small (narrow), light from the light source converges in that narrow region, so there are problems in that the temperature of that region increases, and warping attributable to this rise in temperature occurs.
In addition, when the width dimension of the arcuate effective image forming region is small, the ranges of the light beams that are reflected by the respective reflecting mirrors of the projection optical system also become relatively narrow, so there is a possibility that thermal warping of the respective reflecting mirrors will also become a problem. In particular, to the extent that the intensity of the light that emerges from the light source is strengthened in order to improve throughput, this problem becomes marked, and there is also a possibility that the projection optical system will no longer be able to achieve the desired performance due to an increase in temperature during exposure.